This invention relates to systems and methods for generating pulsed, tunable, monochromatic X-rays. More particularly, this invention pertains to systems for generating pulsed, tunable, monochromatic X-rays with high flux and in a configuration useful both for medical imaging and therapeutics and as a research instrument in the biological, biomedical, and materials sciences.
The characteristics of some X-ray beams are potentially such that they can be used in standard geometry monochromatic imaging, CT-like images of the breast using a rotating mosaic crystal xe2x80x9copticxe2x80x9d time-of-flight xe2x80x9cimaging,xe2x80x9d and phase contrast images. However, X-ray absorption imaging as currently practiced utilizes only a small part of the information amassed by an X-ray beam traversing a patient. For example, assessing damage to limbs and body cavities in severe trauma by appraising the disruption of fascial planes, and visualizing devitalized tissues, extravasated xe2x80x9cblood,xe2x80x9d or imbedded non-opaque foreign materials is very difficult or sometimes impossible with standard X-rays or computerized tomography (CT). The same is true when one wishes to evaluate the patency of arteries and veins, non-invasively and without the use of dangerous contrast agents. Potentially, a great deal more information could be extracted during an examination, if a more versatile monochromatic X-ray beam/detector combination were available for use. Similarly, the early detection of abnormalities such as tumors, fatty replacement, or scarring in other organs such as the breast or lung is problematic at best using conventional imaging techniques and equipment.
Currently, standard X-ray tubes deliver a much broader spectrum of radiation than what is either needed or desired to make an image. Pulsed, xe2x80x9ctunable,xe2x80x9d monochromatic X-rays would allow one to select a photon energy best suited to the imaging task at hand. For example, the frequency that would be optimal to image a breast is very different from the frequency needed to image a chest or the brain.
Monochromatic X-ray imaging can simultaneously reduce the radiation dose to a patient and reduce scattered radiation from high energy photons not needed for the image in the first place. This can be useful in several ways. Cancerous breast tissues, for example, exhibit higher linear attenuation characteristics than do normal tissues, when studied with monochromatic X-rays. This property can be exploited to improve the sensitivity and specificity of breast imaging in a number of ways. The ability to alter the geometry of an X-ray beam would make it ideal for imaging in humans as well as in materials science, molecular biology and cell biology. Standard geometry monochromatic imaging, CT imaging using new X-ray optics made from mosaic crystals, phase contrast imaging, and time-of-flight imaging are just a few examples of the potential applications for such a system.
Conventional medical X-ray equipment has not employed short pulse structures in X-ray generation. Consequently, conventional X-ray equipment continues to generate unneeded background radiation, requiring the use of shielding that substantially increases the size of the equipment. Although pulsed soft X-rays have been used in photolithography for manufacturing integrated circuits, there has been no similar use in imaging applications or in the production of hard X-rays.
Production of pulsed, nearly monochromatic X-rays via the inverse Compton effect (in which optical photons and electrons interact to provide X-ray photons, as demonstrated in FIG. 2 and discussed in more detail below) has been recognized for some time. Systems employing this methodology are theoretically capable of providing a steady supply of ultrashort (e.g., less than 10 ps (picoseconds), X-ray pulse strings. However, such systems exhibit a variety of shortcomings. For example, they typically require large, expensive laser sources to produce the optical photons. Additionally, the systems are unable to adequately control the production of the X-ray pulses, so that appreciable shielding is still required, and any failure of the shielding mechanism may result in a dangerous dose of radiation to a patient. Moreover, the systems are incapable of reducing or eliminating the adverse effects of patient movement during the imaging process. In short, such systems are impractical for wide-spread, convenient use, particularly for the production of high quality, safe X-ray images.
In addition to medical imaging, a source of an intense, pulsed ( less than 10 xe2x80x9cps),xe2x80x9d hard X-rays will be of value in time-resolved structure determination in both materials science and structural biology.
What is needed, then, is a compact source of pulsed, tunable, monochromatic X-rays having the proper beam geometry, low radiation dose, and high brightness to image human beings and other materials.
The problems of prior art X-ray imaging equipment and methods are solved in the present invention of a pulsed monochromatic X-ray system. The X-ray system of the invention is an integrated unit comprised of a conventional tabletop terawatt laser delivering 10 J (joules) of energy in 10 ps at a wavelength of 1.1 microns. The output IR light beam from the laser is counter-propagated against an electron beam produced by a linear accelerator (xe2x80x9cLINACxe2x80x9d) with a photocathode injector and small RF accelerator and gun. X-ray photons are generated by inverse Compton scattering that occurs as a consequence of the xe2x80x9ccollisionxe2x80x9d that occurs between the electron beam and IR photons generated by the laser.
The system uses a novel pulse structure comprising, in a preferred embodiment, a single micropulse. The electron beam from an RF electron LINAC comes in bunches spaced at the RF frequency or some sub-harmonic thereof. These bunches are called microbunches. The light produced by a microbunch (and sometimes the microbunch itself) is called a micropulse. The LINAC is configured to generate an electron beam having 1nC (nanocoulomb) of charge in a microbunch having a pulse length of about 10 picoseconds or less (or an electron beam brightness of 1012 A/m2xe2x80x94radian2@ 500 A). Operating the system in such a single pulse xe2x80x9cmicrobunchxe2x80x9d mode will reduce the need for shielding so that the system can be operated in an environment that is outside of a standard accelerator vault. Accordingly, the system is fabricated in such a way as to fit into a standard sized X-ray room.
A beam alignment xe2x80x94sub-system is used at the IRxe2x80x94electron beam interaction zone and directs the X-ray beam, in a preferred embodiment, through a beryllium window and onto mosaic crystals which divert the beam into a beam transport system toward the imaging target.
The reduction in the amount of shielding required by the system facilitates a configuration in which the X-ray beam deflects off of the mosaic crystals at shallow angles, allowing production and delivery of hard X-rays in the 10 to 50 keV range at high flux (for example, 1.0xc3x971010 photons/pulse). These can be delivered into several adjacent patient examining rooms for use in mammography, plain films of extremities and spine, chest X-rays, abdominal films, CT of all body parts using mosaic crystal rotators, and for angiography and myelography. In addition, the system can be used for time-of flight (xe2x80x9cTOFxe2x80x9d) imaging, phase contrast imaging and weighted sums analysis of tissues, and in radiotherapy and chemoradiotherapy by tuning to K-edges.
A novel feature of the present invention is that the user can obtain an image of human tissue in one shot having a duration of 2-10 ps. Also, because the system operates in the microbunch mode, its physical size is substantially reduced as compared to prior art systems. The reduced background radiation generated by the accelerator makes the system usable in a conventional hospital treatment area or research lab. The system is also inherently safer when running in the microbunch mode in the event of a micropulse of electrons getting out of control due to a system failure. The radiation that a patient would receive, if it were possible for them to receive the radiation from the entire electron bunch, would be about 0.4 to 4 rads, delivered to a very small area. The short pulse duration also eliminates the effects of movement by or within the subject during the imaging process.
In high flux applications, the beams can be split, up to ten times for example, allowing for ten views to be obtained simultaneously in a one-shot CT of 2-10 ps.
Because the system is tunable, an X-ray wavelength can be selected that is most suited to a specific imaging task. For example, the optimal wavelength for imaging a breast is quite different from the optimal wavelengths for imaging the chest or brain. In addition, the X-rays generated by this system are inherently of narrow bandwidth as opposed to the relatively continuous broad spectrum X-rays produced by conventional X-ray tubes. The narrow bandwidth and tunability improve tissue discrimination and allow for improvements in contrast resolution, spatial resolution, and temporal resolution for all procedures.
The system of this invention produces a small effective focal spot size. Consequently, the X-rays can be used in phase contrast imaging, which delivers 100 to 1000 times more information than is available from conventional absorption imaging. The beam geometry of this system also allows for the study of large body parts.
The system can be used with conventional X-ray detectors, such as film, charge coupled devices, and time-of-flight detectors, or with special detectors optimized for use with the characteristics of the X-ray beam and application.
The system can operate in a variety of modes, including:
Plain films, computed radiography, and direct digital radiography to obtain chest radiographs, mammograms, extremity films, spine films, and abdominal films;
Contrast enhanced studies, with K-edge imaging being feasible in both standard angiographic format and with CT techniques, thereby reducing the radiation dose to the patient and while decreasing contrast medium load;
CT and microtomography, where computed tomography yielding 3-D reconstructions of anatomy anywhere in the body, perhaps followed by microtomography of identified lesions:
Weighted sums analysis, where a lesion detected by the system can be analyzed in vivo using a weighted sums analysis of the differential absorption of an area relative to other tissues or to expected norms for that tissue, during multiple exposures made while incrementally changing the beam energy;
Time of flight (TOF) imaging, performed in 2 ps using the monochromatic X-rays generated by this system and eliminating scatter so that the dose may be reduced as compared to using monochromatic beams without TOF techniques; and
Phase contrast imaging, for determining the specific gravity of tissues, detecting infection, tumors and traumatic disruption of tissue planes, and study of blood flow without use of contrast agents.